The present invention relates to the inlet shroud of a gas turbine engine and is specifically addressed to turbofan engines in which the shroud serves as a flow splitter for separating airflow into the compressor inlet and the fan duct.
A shroud or cowling surrounding the inlet of a gas turbine engine defines the flow path for air or other working medium ingested into and diverted around the compressor section of the engine and also may serve a structural function by supporting inlet guide vanes which produce a portion of the engine thrust in most present-day turbomachines for aircraft. During the various modes of engine operation and various flight conditions, the inlet shroud experiences a variety of loads some of which are due to steady-state conditions and some of which are due to transients. Where the shroud is employed to support the inlet guide vanes of a compressor, blow-off or thrust loads must be transmitted continuously through the shroud but vary in accordance with the power output of the engine. Furthermore, transient thermal stresses due to engine and aircraft performance impose additional burdens on the shroud especially due to different flow conditions that exist over the inner and outer walls of the shroud. For example, air being ingested into the gas flow path through the compressor has a different thermal effect on the inner wall of the shroud than the air flowing over the outer wall defining the innermost part of the fan duct in a turbofan engine. Startup and acceleration and deceleration of the engine during changing load demands add further to the thermal transients which exist in this portion of the engine.
The various loads and stresses applied to the inlet shroud present numerous design problems especially when the shroud is comprised of a plurality of individual components in assembled form. Stresses imposed on rivets, screws or bolts which hold the components together result in fatigue failure after extended periods of exposure, and shorten the useful lifetime of the engine. Also, the walls which extend axially of the engine are frequently formed by a series of axially adjacent rings or sections which must not separate when subjected to the various loads and thermal stresses associated with engine operation. Tolerance buildup for serially connected components of this type must also be accounted for since the cumulative effect of deflections due to loading and tolerances may produce an unwanted separation of components.
While most of the above problems can be addressed by heavier and additional fasteners as well as by thicker wall sections, the net result is an overall increase in engine weight and cost and a reduction in the power-to-weight ratio. It is, therefore, preferable that the problem be addressed by improved design which does not impose performance penalties nor increase engine cost.
It is, accordingly, a general object of the present invention to provide an inlet shroud or flow splitter which accommodates the various loads and stresses by virtue of its design and without the penalties of additional weight or more exotic and costly materials. It is a further object of the invention to provide a shroud which accommodates any tolerance buildup that alone or in combination with applied loads might separate the shroud components under the stress and strain of flight conditions.